The present invention relates to a method and apparatus for the continuous production of tubing, rods and the like from crystalline quartz or other glass like materials. More particularly, this invention relates to a crucible for use in the production of elongated quartz members from a silica melt.
Various elongated members have been formed continuously by melting of quartz crystal or sand in an electrically heated furnace whereby the desired shape is drawn from the furnace through a suitable orifice or die in the bottom of the furnace as the raw material is melted. One apparatus for continuous production of fused quartz tubing, for example, is a tungsten-lined molybdenum crucible supported vertically and having a suitable orifice or die in the bottom to draw cane, rods, or tubing. The crucible is surrounded by an arrangement of tungsten heating elements or rods which heat the crucible. The crucible, together with its heating unit, is encased in a refractory chamber supported by a water-cooled metal jacket. The crucible is heated in a reducing atmosphere of nitrogen and hydrogen. Because tungsten is transported into the melt, it is important to maintain a relatively low temperature of about 2000xc2x0 C.
An alternative apparatus provides clear fused quartz tubing by feeding natural quartz crystal into a refractory metal crucible heated by electrical resistance under a unique gas atmosphere to reduce the bubble content. The bubbles formed by gas entrapment between crystals and the molten viscous mass of fused quartz do not readily escape from the molten glass and, hence, remain as bubbles or ridges in the product drawn from the fused quartz melt. By substituting a melting atmosphere gas which readily diffuses through the molten material (such as pure helium, pure hydrogen or mixtures of these gases) it was possible to reduce the gas pressure in the bubbles and thereby reduce the bubble size. This process uses a mixture of 80% helium and 20% hydrogen by volume.
In a further alternative method, a product is obtained by continuously feeding a raw material of essentially pure silicon dioxide in particulate form into the top section of an induction-heated crucible, fusing the raw material continuously in an upper-induction heat zone of the crucible in an atmosphere of hydrogen and helium while maintaining a fusion temperature not below approximately 2050xc2x0 C. The fused material in the lower zone of the crucible is heated by separate induction heating means to produce independent regulation of the temperature in the fused material. The fused material is continuously drawn from the lower zone of the crucible through forming means in the presence of an atmosphere of hydrogen containing a non-oxidizing carrier gas.
Unfortunately, most of the refractory metals and non-metal materials used in the crucibles of the above-described apparatus react with silica at high temperatures. At these temperatures, oxides of the refractory materials dissolve and diffuse into the silica and contaminate the glass. Such refractory material contamination causes discoloration and occlusions in the silica glass fused in crucibles made of such refractory materials. For example, refractory materials used in traditional crucibles leave at least from 12-300 ppb of the refractory materials in the silica melt. Accordingly, there is a need in the art to reduce contamination of fused glass occurring from the refractory materials. This need has increased recently as semiconductor and fiber optics manufacturing processes, a primary use for the glass products obtained from the subject production process, have required higher levels of purity.
Furthermore, the amount of refractory metal in the silica glass melt is believed to be proportional to the fusion temperature. Therefore, unless a very strict control over the furnace operating temperature is exercised, levels of refractory metal contamination can easily become unacceptable. Of course, such strict temperature operational limits imposed on the furnace operation are problematic. In fact, strict temperature limits can detract from a typical need for the higher fusion temperatures which are used to achieve better visual characteristics in the resultant fused quartz product.
In an exemplary embodiment- of the invention, a crucible for melting of silica and subsequent drawing into a desired shape is comprised of a body having an outer surface constructed of a refractory material and including a inner lining of a non-reactive barrier material selected from rhenium, osmium, iridium or mixtures thereof In a preferred embodiment, the non-reactive barrier lining between 0.010xe2x80x3 and 0.050xe2x80x3 in depth.
The present crucible construction provides a number of advantages over the prior art. Particularly, furnaces constructed with rhenium, iridium, and/or osmium lined crucibles produce products with much lower levels of refractory metal in the solution. For example, the metal dissolved in the silica can be reduced to below 10 ppb, preferably below 1 ppb, and preferably below the current level of detection via NAA. This reduced amount of refractory metal contamination in the silica melt improves the chemical composition of the silica glass allowing for a decrease in discoloration and surface haze. Furthermore, utilization of a furnace equipped with a crucible including the non-reactive lining allows operation at optimum temperature ranges. More specifically, the non-reactive crucible allows the furnace to operate at temperatures in excess of 2350xc2x0 C. Operation at these higher temperatures achieves better fining. Moreover, operation at optimum fusion temperatures will increase solubility of gaseous species in the raw material, thus reducing airline defects in the drawn products. Similarly, the present inventive crucible will also help to further reduce the presence of haze and discoloration in the resultant glass products.
It should be noted that the terms xe2x80x9cquartzxe2x80x9d and xe2x80x9csilicaxe2x80x9d are used interchangeably throughout this application, both being directed generally to the compound SiO2. Similarly, the present invention encompasses the use of any raw material introduced to the melting furnace, including but not limited to natural silica/quartz and synthetic silica.